Stacked type falling film evaporator, zero liquid discharge system comprising the same, and zero liquid discharging method using the same

ABSTRACT

A stacked type falling film evaporator includes a first evaporator, a second evaporator, a first vapor recovering device, a second vapor recovering device and a vapor recompressor. The first evaporator and the second evaporator respectively have evaporation tubes of a length of 5 m to 10 m, and are stacked in such a manner that wastewater passes through the first evaporator and the second evaporator in order. The first vapor recovering device collects vapor generated from the wastewater in the first evaporator and supplies the collected vapor to the second evaporator. The second vapor recovering device collects vapor generated from the wastewater in the second evaporator and supplies the collected vapor to the first evaporator. The vapor recompressor compresses the vapor collected in the second vapor recovering device before the vapor is supplied to the first evaporator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Application No.10-2014-0002462, filed Jan. 8, 2014, the contents of which areincorporated herein in their entirety.

BACKGROUND

The present disclosure relates to a stacked type falling filmevaporator, a zero liquid discharge system comprising the same, a zeroliquid discharging method using the same. In particular, two evaporatorsmay respectively have evaporation tubes with a relatively short lengththat are installed in a two-stage stacked type evaporator in a verticaldirection, thereby addressing the problems of evaporators with reducedefficiency due to formation of a dry zone inside the evaporation tubes,providing easy installation and maintenance due to modularization,simplifying pipelines, and reducing a site area required for installingequipment.

The stacked type falling film evaporator may be applied to a zero liquiddischarge system for desulfurized wastewater in power plants orindustrial wastewater, but is not restricted to the above and may beapplied to various industry fields where a high-degree evaporationprocess, seawater desalination, or salt production processes are useful.

Recently, companies and academic circles are becoming more and moreinterested in Zero Liquid Discharge (ZLD) systems. There are indicationsof a gradual increase of the supply price of industrial water, a gradualincrease of production costs due to an increase of discharge fees byregulations on total quantity of effluent water, and a scheduledenactment of a recycling obligation of more than ⅓ of effluent dischargeflow. Also, companies and academic circles value judgments are changingto avoid concerns over environmental and civil concerns. Recently, therehas been a movement to introduce zero liquid discharge systems not onlyfor specific wastewater but also for all water.

It is estimated that approximately 100 zero liquid discharge systems arenow operational in Japan, and thousands of zero liquid discharge systemsare operational in the United States.

The zero liquid discharge systems in Japan are mainly installed in highvalued added semiconductor factories but require a great deal of highquality water and have significant installation and operating expensesbecause the installation regions are limited to specific contaminantdischarge areas, such as national parks.

For example, the Canon company is located in Oita, Japan andmanufactures cartridges for photocopiers. Oita is incorporated in alimited immutable weight region and a ban was requested on wastewaterdischarge by the nearby National Federation of Fisheries Cooperatives.Hence, the Canon company introduced a zero liquid discharge system. Asanother example, UMC Japan, which is a semiconductor manufacturingfactory, introduced a zero liquid discharge system because the factorywas located in a national park.

Zero liquid discharge systems in the United States have been introducedmainly in areas where strict effluent water quality standards wereapplied, when effluent water quality standards were established by thestate, or in factories located in regions lacking an abundant supply ofwater such as deserts.

As an example, La Paloma Plant is a thermoelectric power plant locatedin the middle of the Mojave Desert in California. It introduced a zeroliquid discharge system because very strict effluent water qualitystandards were applied. The plant is located in a large-scaleagricultural area, water supply from nearby areas of the Mojave Desertis not good, and the price of water is very high. La Paloma Plant reusesrecovered water as boiler makeup water for running a power plantturbine. As another example, Intel, which is a semiconductor factory,also introduced a zero liquid discharge system because of stricteffluent water quality standards and industrial water shortages in theclimate of Arizona.

This kind of ZLD, which is a process of reusing treated sewage water andof discharging a small quantity of sludge excluding the treated water,is divided into two types. A Reverse Osmosis (RO) ZLD is a separationprocess using reverse osmosis. A thermal ZLD evaporatively concentratesand phase changes by heating. Evaporative concentration technologyutilized in the 19^(th) century food industry and intensifiedenvironmental regulations leading to increased reuse of water resourceshave increased demands in thermal ZLD technology applied in variousindustry fields.

The thermal ZLD process using phase change be heating is mostlyeffective in non-degradable wastewater. A waste heat steam heating typeuses a boiler, an evaporation type operates in a vacuum decompressedstate, and a flame direct heating type uses methods of heatingwastewater.

The waste heat steam heating type requires a large-scale infrastructure,a large installation scale and excessive installation costs. A flamedirect heating type provides a treatment effect like a simpleevaporation type but has increased energy consumption.

The vacuum decompressed evaporation type provides an energy savingsbecause it is a method of evaporation by lowering the boiling point ofwastewater while keeping the pressure inside the evaporator in a vacuumcondition. But this type has several disadvantages including that thetreatment result may be non-uniform dependent on conditions of theintroduced wastewater, and that it is difficult to maintain and repairdue to maintenance of the vacuum condition.

A schematic basic structure of a falling film evaporator is illustratedin FIG. 1.

The evaporator 1 includes: a cylindrical housing 10; a flow uniformitydevice 20 provided by a plate horizontally mounted at an upper portionof the housing 10; and a wastewater inlet 11 mounted higher than theflow uniformity device 20, such that introduced wastewater is suppliedto an upstream space S1 of the upper portion of the flow uniformitydevice 20.

A plurality of evaporation tubes 30 are densely mounted inside thehousing 10. Upper ends of the evaporation tubes 30 penetrate the flowuniformity device 20 in such a way that the upstream space S1communicates with an inner space of the evaporation tubes 30. Therefore,wastewater supplied into the upstream space S1 flows down along theinner walls of the evaporation tubes 30. The wastewater evaporated andconcentrated while passing through the evaporator 1 is collected in aliquid storage tank S3 of the evaporator 1. The wastewater flowing downalong the inner walls of the evaporation tubes 30 forms a falling filmthat is evaporated while being heated by heat exchange with vapordisposed on the outside of the walls of the evaporation tubes 30. Thevapor is introduced into a heat exchange space S2 in the middle of thehousing 10 via a vapor inlet 40. A vapor outlet 50 extracts vapor havingundergone heat exchange in the heat exchange space S2 of the housing 10and discharges the extracted vapor from the evaporator 1.

Such a falling film evaporator may restrain a rise of the boiling pointwithout pressure loss inside the device and reduce contact time with aheating body, such as vapor, because a heat transfer rate is very higheven though there is a difference in temperature with a heating fluid.Moreover, the falling film evaporator may reduce temperature increase ofliquid that is sensitive to heat because a thermal gradient in a liquidfilm kept at about 1 mm to 2 mm is very small.

Furthermore, the falling film evaporator may sufficiently transfer heateven though a temperature difference between the heating fluid and theinside fluid becomes less than 10° C. because the heat transfer rate isvery high. The inside fluid temperature may be maintained and not riseexcessively.

The falling film evaporator has further advantages in that a period oftime required for achieving steady-state operating conditions of theevaporator is relatively short and a period of time required forstopping operation is also short because the volume of the wastewaterinside the tube is small. It is effective in low enriched fluidevaporation and power consumption of a circulation pump may be kept low.

However, if the wastewater film is broken, scale may be formed on theinner walls of the tubes. The falling film evaporator is difficult toemploy when the wastewater includes lots of scale components. It alsomay require a distributor design technology to distribute wastewater toeach of the tubes uniformly, for example, to prevent the film frombreaking.

In more detail, the falling film evaporator includes long tubes of morethan 10 m in length. Therefore, the falling film evaporator has severaldisadvantages in that an evaporative concentration efficiency of theentire device is reduced because a zone where the wastewater film isdried, namely a dry zone, is highly likely to be formed in the proximityof the downstream side, in that workability is deteriorated at the timeof installation or maintenance, and in that it takes a relatively longtime to achieve operational conditions because the tubes are long.

BRIEF SUMMARY

Accordingly, the present disclosure has been made to address theabove-mentioned problems. It is an object of the present disclosure toprovide a stacked type falling film evaporator, a zero liquid dischargesystem comprising the same, and a zero liquid discharging method usingthe same. Two evaporators may respectively have evaporation tubes with arelatively short length that are installed in a two-stage stacked typein a vertical direction, thereby addressing the problems of otherevaporators with reduced efficiency due to formation of a dry zoneinside the evaporation tubes, providing easy installation andmaintenance due to modularization, simplifying pipelines, and reducing asite area required for installing equipment.

In an embodiment, there is provided a stacked type falling filmevaporator which includes: a housing standing erect and having awastewater inlet disposed at an upper portion thereof and a liquidstorage tank disposed at a lower portion thereof for storingconcentrated wastewater; a flow uniformity device horizontally fixed andmounted at the upper portion of the housing in such a manner thatwastewater introduced from the wastewater inlet can be stored; aplurality of evaporation tubes standing erect in such a manner that theupper end vertically penetrates the flow uniformity device and anopening portion of the upper end communicates with an upstream space ofthe flow uniformity device; a vapor inlet for introducing vapor into aheat exchange space of the housing in order to heat the outer walls ofthe evaporation tubes; and a condensate water recovery hole forrecovering condensate water which is introduced into the heat exchangespace and condensed through heat exchange with evaporation tubes, suchthat the wastewater introduced through the wastewater inlet flows intothe upstream space and is evaporated while flowing in the form of afalling film along the inner walls of the evaporation tubes, the fallingfilm evaporator further including: a first evaporator and a secondevaporator respectively having evaporation tubes of a length of 5 m to10 m, the first evaporator and the second evaporator being stackedvertically in such a manner that wastewater passes through the firstevaporator and the second evaporator in order; a first vapor recoveringdevice for collecting vapor generated from the wastewater in the firstevaporator and supplying the collected vapor to the second evaporator; asecond vapor recovering device for collecting vapor generated from thewastewater in the second evaporator and supplying the collected vapor tothe first evaporator; and a vapor recompressor for compressing vaporbefore the vapor collected in the second vapor recovering device issupplied to the first evaporator.

The vapor recompressor may be a thermal vapor recompressor, and thestacked type falling film evaporator may include second vaporrecompressor 170 for compressing vapor before the vapor collected in thefirst vapor recovering device is supplied to the second evaporator.

The stacked type falling film evaporator may include a circulation pumpfor supplying the concentrated wastewater passing through the secondevaporator to the upper portion of the first evaporator. When thefalling film evaporator is operated, the temperature of the wastewaterinside the evaporation tube may be 70° C. to 130° C. and the insidepressure of the evaporation tube may be 50 Torr to 150 Torr.

In another embodiment, a zero liquid discharge system includes a fallingfilm evaporator for evaporating wastewater introduced through awastewater inlet while the introduced wastewater flows into the upstreamspace and flows in the form of a falling film along the inner walls ofevaporation tubes, the zero liquid discharge system further including: awastewater pretreatment device in which the introduced wastewater passesin consecutive order; a crystallization device; a condenser adapted forfinally condensing vapor generated from the wastewater in order torecover condensate water; and a sludge treatment device for treatingsludge generated from the wastewater into a finally discarded formthrough centrifugation, wherein the falling film evaporator includes: afirst evaporator and a second evaporator respectively having evaporationtubes of a length of 5 m to 10 m, the first evaporator and the secondevaporator being stacked vertically in such a manner that wastewaterpasses through the first evaporator and the second evaporator in order;a first vapor recovering device for collecting vapor generated from thewastewater in the first evaporator and supplying the collected vapor tothe second evaporator; a second vapor recovering device for collectingvapor generated from the wastewater in the second evaporator andsupplying the collected vapor to the first evaporator; and a vaporrecompressor for compressing vapor before the vapor collected in thesecond vapor recovering device is supplied to the first evaporator.

The wastewater pretreatment device may be at least one selected from agroup of a caustic soda treatment tank, an alum treatment tank, apolymer treatment tank, and a settling separation tank, thecrystallizing device is a forced circulation evaporator, and the zeroliquid discharge system further includes a reverse osmosis separatordisposed at the upstream side of the falling film evaporator.

In another embodiment, a zero liquid discharging method includes: apretreatment step of pretreating introduced wastewater through awastewater pretreatment device; an evaporation step of evaporating andconcentrating the pretreated wastewater through a falling filmevaporator; a crystallizing step of phase-separating the concentratedwastewater through a crystallizing device; and a post-treatment step offinally concentrating vapor, which is phase-separated and generated fromthe wastewater, through a condenser so as to recover condensate waterand treating the sludge generated from the wastewater into a finallydiscarded form through centrifugation by a sludge treatment device,wherein the evaporation step includes: a first evaporation step and asecond evaporation step of evaporating and concentrating wastewaterintroduced into the first evaporator in order through a falling filmevaporator in which a first evaporator and a second evaporatorrespectively having evaporation tubes of a length of 5 m to 10 m arestacked vertically; a downward supplying step of collecting vaporgenerated from wastewater in the first evaporator and supplying thecollected vapor to the second evaporator through a first vaporrecovering device; an upward recompressing step of collecting vaporgenerated from the wastewater in the second evaporator and transferringthe collected vapor to a vapor recompressor through a second vaporrecovering device; and an upward supplying step of supplying the vaporcompressed in the vapor recompressor to the first evaporator.

The zero liquid discharging method may include: a downward recompressingstep of compressing vapor generated from the wastewater in the firstevaporator through second vapor recompressor 170 before supplying thecollected vapor to the second evaporator in the downward supplying step;or a wastewater circulating step of supplying the concentratedwastewater passing through the second evaporator to the upper portion ofthe first evaporator through a circulation pump after the secondevaporation step.

The wastewater pretreatment device in the pretreatment step may be atleast one selected from a group of a caustic soda treatment tank, analum treatment tank, a polymer treatment tank, and a settling separationtank. The zero liquid discharging method may include an RO (ReverseOsmosis) step of separating the pretreated wastewater through a reverseosmosis separator.

The stacked type falling film evaporator, the zero liquid dischargesystem comprising the same, and the zero liquid discharging method usingthe same according to the embodiments of the present disclosure mayenhance workability in installation and maintenance and may arrive atsteady-state operating conditions because the evaporation tubes areshorter than the long tubes mounted in other falling film evaporators.

Additionally, the stacked type falling film evaporator, the zero liquiddischarge system comprising the same, and the zero liquid dischargingmethod using the same according to the embodiments of the presentdisclosure may address problems in installation and maintenance due tomodularization of the stacked type system units, reduce required sitearea because the falling film evaporator is the stacked type, reduceenergy consumption for carrying out processes because the pipingstructure of the falling film evaporator is simpler than a horizontaltype multi-stage evaporator and does not require additional energyconsumption for supply of wastewater to the downstream side, andmultilaterally cope with the problem of the heat exchange typeevaporator by vertical tubes that scale is formed on the inner walls ofthe tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will be apparent from the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional diagram showing a structure of a fallingfilm evaporator;

FIG. 2 is a cross-sectional diagram showing a structure of a stackedtype falling film evaporator according to an embodiment of the presentdisclosure;

FIG. 3 is a system diagram showing a zero liquid discharge systemincluding the stacked type falling film evaporator according to anotherembodiment of the present disclosure; and

FIG. 4 is a flow chart showing a zero liquid discharge method using thestacked type falling film evaporator according to a further embodimentof the present disclosure.

DETAILED DESCRIPTION

Reference will be now made in detail to the embodiments of the presentdisclosure with reference to the attached drawings. It will beunderstood that words or terms used in the specification and claimsshall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of thedisclosure.

Referring to FIG. 2, according to an embodiment of the presentdisclosure, a stacked type falling film evaporator includes a firstevaporator 110 and a second evaporator 120 respectively havingevaporation tubes 30 of a length of 5 m to 10 m. The first evaporator110 and the second evaporator 120 are stacked vertically in such amanner that wastewater passes through the first evaporator 110 and thesecond evaporator 120 in order. A first vapor recovering device 130collects vapor generated from wastewater in the first evaporator 110 andsupplies the collected vapor to the second evaporator 120. A secondvapor recovering device 140 collects vapor generated from wastewater inthe second evaporator 120 and supplies the collected vapor to the firstevaporator 110. A vapor recompressor 150 compresses vapor before thevapor collected in the second vapor recovering device 140 is supplied tothe first evaporator 110.

The plurality of evaporation tubes 30 inserted into the first evaporator110 and the second evaporator 120 of the falling film evaporator 100 mayhave a length of 5 m to 10 m. As described above, evaporation tubes ofother falling film evaporators are generally more than 10 m long, andhence, have the above-mentioned problems.

If the evaporation tubes 30 are less than 5 m long, the tubes may be tooshort and a falling film formed on the inner wall of each of theevaporation tubes 30 may be thick and it is difficult to control thefalling film to the optimum thickness. However, if the evaporation tubes30 are more than 10 m long, efficiency in an evaporation process may bedeteriorated due to formation of a dry zone at a lower end of the tube.

As described above, because the evaporation tube 30 is short, thefalling film evaporator may enhance workability in installation ormaintenance/repair and easily achieves steady state operatingconditions.

The quantity of wastewater may be reduced by shortening the evaporationtube 30. The first and second evaporators may be stacked vertically insuch a manner that wastewater is evaporated and concentrated whilepassing the first evaporator 110 and the second evaporator 120 in order.

Therefore, the stacked type falling film evaporator according toembodiments of the present disclosure may increase an amount ofwastewater that will be treated in the evaporation process, which mayaddress problems in installation and maintenance due to modularizationof the stacked type system units, reduce a required site area becausethe falling film evaporator is the stacked type, reduce energyconsumption for carrying out processes because the piping structure ofthe falling film evaporator is simpler than the horizontal typemulti-stage evaporator and does not require additional energyconsumption for supply of wastewater to the downstream side, andmultilaterally copes with the problem of the heat exchange typeevaporator by vertical tubes that scale is formed on the inner walls ofthe tubes.

Moreover, the stacked type falling film evaporator may include a firstvapor recovering device 130, a second vapor recovering device 140, and avapor recompressor 150. The wastewater introduced into the firstevaporator 110 is heated and evaporated in the evaporation tubes 30,such that vapor is generated. The vapor generated from wastewater iscollected, and then, is not directly condensed and discharged buttransferred to the second evaporator 120 so as to be used for heatingwastewater. For this, the first vapor recovering device 130 is provided.Furthermore, the second evaporator 120 also generates vapor fromwastewater, and the vapor is collected and transferred to the firstevaporator 110 through the second vapor recovering device 140. In thisinstance, when the vapor collected from the second evaporator 120 iscompressed into high-pressure vapor and supplied to the first evaporator110, the recompressor 150 enhances heat-exchange efficiency of thevapor.

The vapor recompressor 150 serves to compress the vapor collected fromthe evaporator into high-pressure vapor, and may be provided by amechanical vapor recompressor and/or a thermal vapor recompressor.According to an embodiment of the present disclosure, the vaporrecompressor 150 may be a thermal vapor recompressor. The thermal vaporrecompressor may be more energy efficient, may process more vapor thanthe mechanical vapor recompressor, and may be more effectively appliedto the stacked structure of the falling film evaporator 100. However, amechanical vapor recompressor may also be used.

In addition to a vapor recompressor 150 coupled to pipes which collectthe vapor from the second evaporator 120 and supply the compressed vaporto the first evaporator 110, second vapor recompressor 170 is coupled topipes which collect vapor from the first evaporator 110 and supply thecompressed vapor to the second evaporator 120 so as to enhance theheat-exchange efficiency in the second evaporator 120. Additionally,when the operation of the two vapor recompressors 150, 170 iscontrolled, improved vapor circulation may be achieved.

The wastewater evaporated and concentrated while passing through thefirst evaporator 110 and the second evaporator 120 in order is collectedin a liquid storage tank S3 of the second evaporator 120. A fixed amountof the collected and concentrated wastewater is supplied to a wastewaterinlet 11 located at an upper portion of the first evaporator 110 througha circulation pump 160 in order to increase the concentration ofwastewater which will be treated under the steady-state operatingconditions.

During operation of the falling film evaporator 100, the temperature ofthe wastewater inside the evaporation tubes 30 and the inside pressureof the evaporation tubes 30 may be controlled according to physicalconditions, such as ingredients, viscosity and quantity of thewastewater. In some cases, the temperature of the wastewater inside theevaporation tubes 30 is 70° C. to 130° C. and the inside pressure of theevaporation tubes 30 is 50 Torr to 150 Torr.

When the temperature of vapor supplied to the evaporator is controlledto 10° C. to 120° C., temperature of the wastewater inside the systemmay be controlled to 70° C. to 130° C. and the inside pressure of thesystem may be controlled to 50 Torr to 150 Torr so as to improve heattransfer efficiency and reduce chemical reactions between differentingredients contained in the wastewater.

If the temperature of the wastewater is lower than 70° C., thewastewater is not effectively evaporated and evaporation efficiency ofthe entire system is deteriorated. Furthermore, if the temperature ofthe wastewater exceeds 130° C., chemical reactions may occur between thedifferent components in the wastewater, and the invested thermal energyis not economical.

Additionally, if the inside pressure of the evaporation tubes 30 is lessthan 50 Torr, a high vacuum condition is kept, which is not energyefficient and causes difficulty in operating processes. If the insidepressure of the evaporation tubes 30 exceeds 150 Torr, evaporation andseparation efficiencies may be deteriorated.

FIG. 3 is a system diagram showing the zero liquid discharge system 200according to an embodiment of the present disclosure.

Referring to FIG. 3, according to another embodiment of the presentdisclosure, a zero liquid discharge (ZLD) system includes: a wastewaterpretreatment device 210 in which introduced wastewater passes inconsecutive order, a falling film evaporator 100, a crystallizationdevice 230, a condenser 240 adapted for condensing vapor generated fromthe wastewater in order to recover condensate water, and a sludgetreatment device 250 for treating sludge generated from the wastewaterinto a discarded form through centrifugation.

The falling film evaporator 100 includes a first evaporator 110 and asecond evaporator 120 respectively having evaporation tubes 30 of alength of 5 m to 10 m. The first evaporator 110 and the secondevaporator 120 are stacked vertically in such a manner that wastewaterpasses through the first evaporator 110 and the second evaporator 120 inorder. A first vapor recovering device 130 collects vapor generated fromwastewater in the first evaporator 110 and supplies the collected vaporto the second evaporator 120. A second vapor recovering device 140collects vapor generated from wastewater in the second evaporator 120and supplies the collected vapor to the first evaporator 110. A vaporrecompressor 150 compresses vapor before the vapor collected in thesecond vapor recovering device 140 is supplied to the first evaporator110.

The zero liquid discharge system may be a thermal Zero Liquid Discharge(ZLD) system or an Reverse Osmosis (RO) ZLD system. In the describedembodiment, the zero liquid discharge system 200 is a thermal ZLDsystem. As described above, the zero liquid discharge system 200includes the wastewater pretreatment device 210, the falling filmevaporator 100, the crystallization device 230, the condenser 240, andthe sludge treatment device 250.

The wastewater pretreatment device 210 is a device for removing andseparating relatively large impurities through precipitation orprecipitation-related response. Furthermore, pretreatment devices ofvarious types may be adopted according to physical properties ofwastewater. A caustic soda treatment tank 211 may be used to neutralizeacid wastewater, such as desulfurized wastewater, using caustic soda. Analum treatment tank 212 may be used to separate organic materials usinga coagulating agent, such as alum. A polymer treatment tank 213 may beused to deposit floating matter through a polymer coagulant. A settlingseparation tank 214 may be used to separate deposits by storing andchemically treating wastewater for a predetermined period of time.

The crystallization device 230 is a device used for a crystallizingprocess in which high-temperature evaporation concentrates wastewater,which was evaporated and concentrated while passing the falling filmevaporator 100, so as to separate the wastewater into solid-phase sludgeand vapor.

Various crystallizing devices 230 may be used. In the describedembodiment, a forced circulation evaporator is used. The forcedcirculation evaporator uses a pump that is wider in installation areathan the falling film evaporator 100 to keep the velocity inside theevaporation tubes 30 at 2 m/s to 3 m/s, which may have high powerconsumption and require expensive equipment. However, the forcedcirculation evaporator has several advantages in that scale formationrate is low because it can keep a liquid flow steady inside the heatexchange tube to maintain the liquid film thickness. It is easy toclean, and it can accommodate high-concentration fluids or fluids inwhich scale is formed easily. It is suitable for treatment ofhigh-concentration wastewater with a high Boiling Point Elevation (BPE)and wastewater which is difficult in natural circulation due to a highviscosity. Therefore, the forced circulation evaporator is suitable foradditionally evaporating and concentrating highly concentratedwastewater which passes the falling film evaporator 100.

Next, after the crystallizing process, the wastewater passes to thecondenser 240 and the sludge treatment device 250. A phase separation ofthe concentrated wastewater occurs through the crystallizing process inthe crystallizing device 230. In this instance, vapor generated from thewastewater is collected, and then, the collected vapor is cooled intocooling water by the condenser 240 and is discharged or is generatedinto reusable condensate water. Meanwhile, the sludge generated from thewastewater is treated into a sludge cake through centrifugation in thesludge treatment device 250 and then discharged.

Moreover, the zero liquid discharge system may also include a reverseosmosis separator 220 disposed at the upstream side of the falling filmevaporator 100. In order to increase zero liquid discharge efficiency ofthe wastewater, the present disclosure may adopt a hybrid process thatthe ZLD process by reverse osmosis is combined to the thermal ZLDprocess of the present disclosure.

FIG. 4 is a flow chart showing the zero liquid discharge methodaccording to the embodiment of the present disclosure.

Referring to FIG. 4, according to a further embodiment of the presentdisclosure, a zero liquid discharging method 300 includes: apretreatment step (S10) of pretreating introduced wastewater through awastewater pretreatment device 210; an evaporation step (S30) ofevaporating and concentrating the pretreated wastewater through afalling film evaporator 100; a crystallizing step (S40) ofphase-separating the concentrated wastewater through a crystallizingdevice 230; and a post-treatment step (S50) of concentrating vapor,which is phase-separated and generated from the wastewater, through acondenser 240 so as to recover condensate water and treating the sludgegenerated from the wastewater into a discarded form throughcentrifugation by a sludge treatment device 250. The evaporation step(S30) includes: a first evaporation step (S31) and a second evaporationstep (S32) of evaporating and concentrating wastewater introduced into afirst evaporator 110 and a second evaporator 120 of a falling filmevaporator, in order, respectively having evaporation tubes 30 of alength of 5 m to 10 m and stacked vertically; a downward supplying step(S34) of collecting vapor generated from wastewater in the firstevaporator 110 and supplying the collected vapor to the secondevaporator 120 through a first vapor recovering device 130; an upwardrecompressing step (S36) of collecting vapor generated from thewastewater in the second evaporator 120 and transferring the collectedvapor to a vapor recompressor 150 through a second vapor recoveringdevice 140; and an upward supplying step (S37) of supplying the vaporcompressed in the vapor recompressor 150 to the first evaporator 110.

Hereinafter, the zero liquid discharging method 300 will be described intime sequential order.

First, wastewater is introduced into the wastewater pretreatment device210, and then, goes through the pretreatment step (S10). After that, thepretreated wastewater may go through the evaporation step (S30) in thefalling film evaporator 100 or may go through an RO step (S20) in thereverse osmosis separator 220 according to an embodiment of the presentdisclosure.

To form higher-concentration wastewater by recovering moisture from theconcentrated wastewater through the evaporation step (S30), thewastewater goes through the crystallizing step (S40) in which phaseseparation of the concentrated wastewater occurs in the crystallizingdevice 230 (e.g., a forced circulation evaporator). Vapor and sludgegenerated through the phase separation of the wastewater by thecrystallizing step (S40) are respectively concentrated in the condenser240 or through the post-treatment step (S50) such that the sludge istreated into a discardable form, such as a sludge cake, throughcentrifugation by the sludge treatment device 250.

The evaporation step (S30) may be divided into a flow of wastewater anda flow of vapor/condensate water. First, the flow of wastewater will bedescribed. The wastewater goes through the first evaporator 110 in thefirst evaporation step (S31), and through the second evaporator 120 inthe second evaporation step (S32). Some of the wastewater concentratedafter the second evaporation step (S32) is returned to the firstevaporator 110 in the wastewater circulating step (S33).

Now, the flow of vapor/condensate water will be described. Vaporintroduced into the first evaporator 110 is concentrated through heatexchange with the wastewater inside the evaporation tubes 30 generatingvapor from the wastewater. The generated vapor goes through the downwardsupplying step (S34) of collecting the vapor and supplying the collectedvapor to the second evaporator 120, the upward recompressing step (S36)of collecting vapor generated from the wastewater in the secondevaporator 120 and transferring the collected vapor to the vaporrecompressor 150, and the upward supplying step (S37) of supplying thehigh-pressure vapor generated in the vapor recompressor 150 to the heatexchange space S2 of first evaporator 110. The zero liquid dischargingmethod may further include a downward recompressing step (S35) ofsupplying high-pressure vapor generated by second vapor recompressor 170to the second evaporator 120 after the upward supplying step (S34).

While the present disclosure has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes andmodifications may be made therein without departing from the technicalidea and scope of the present disclosure and such changes andmodifications belong to the claims of the present disclosure. Further,the embodiments discussed have been presented by way of example only andnot limitation. Thus, the breadth and scope of the invention(s) shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents. Moreover, the above advantages and features are provided indescribed embodiments, but shall not limit the application of the claimsto processes and structures accomplishing any or all of the aboveadvantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called technical field.Further, a description of a technology in the “Background” is not to beconstrued as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Brief Summary” to beconsidered as a characterization of the invention(s) set forth in theclaims found herein. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty claimed in this disclosure. Multipleinventions may be set forth according to the limitations of the multipleclaims associated with this disclosure, and the claims accordinglydefine the invention(s), and their equivalents, that are protectedthereby. In all instances, the scope of the claims shall be consideredon their own merits in light of the specification, but should not beconstrained by the headings set forth herein.

What is claimed is:
 1. A stacked type falling film evaporator,comprising: a first falling film evaporator and the second falling filmevaporator stacked in such a manner that wastewater passes through thefirst falling film evaporator and the second falling film evaporator inorder; a first pipe, connected between a liquid storage tank of thefirst falling film evaporator and a vapor inlet of the second fallingfilm evaporator, that collects vapor generated from the wastewater inthe first falling film evaporator and supplies the collected vapor tothe second falling film evaporator; a second pipe, connected between aliquid storage tank of the second falling film evaporator and a vaporinlet of the first falling film evaporator, that collects vaporgenerated from the wastewater in the second falling film evaporator andsupplies the collected vapor to the first falling film evaporator; avapor recompressor that compresses the vapor collected in the secondpipe before the vapor is supplied to the first falling film evaporator;and a circulation pump that supplies concentrated wastewater exiting thesecond falling film evaporator to an upstream space of the first fallingfilm evaporator.
 2. The stacked type falling film evaporator accordingto claim 1, wherein the vapor recompressor is a thermal vaporrecompressor.
 3. The stacked type falling film evaporator according toclaim 1, further comprising a second vapor recompressor that compressesthe vapor collected in the first pipe before the vapor is supplied tothe second falling film evaporator.
 4. The stacked type falling filmevaporator according to claim 1, wherein each of the first and secondfalling film evaporators includes: a housing having a wastewater inletdisposed at a top portion thereof and the liquid storage tank thatstores concentrated wastewater disposed at bottom portion thereof; aflow uniformity device that separates an upstream space from a heatexchange space comprising a plurality of evaporation tubes; theplurality of evaporation tubes of a length of about 5 m to 10 m passingthrough the flow uniformity device such that opening portions of ends ofthe tubes are disposed in the upstream space; the vapor inlet thatintroduces vapor into the heat exchange space in order to heat the outerwalls of the evaporation tubes; and a condensate water recovery aperturethat recovers condensate water which is introduced into the heatexchange space and concentrated through heat exchange with theevaporation tubes, such that the wastewater introduced through thewastewater inlet flows into the upstream space and is evaporated whileflowing in the form of a falling film along inner walls of theevaporation tubes.
 5. A zero liquid discharge system, comprising: astacked type falling film evaporator; a wastewater pretreatment devicein which introduced wastewater passes pretreatment devices inconsecutive order and is passed to the stacked type falling filmevaporator; a crystallization device to concentrate, by high-temperatureevaporation, wastewater which was evaporated and concentrated whilepassing through the stacked type falling film evaporator; a condenseradapted to condense vapor, generated from the wastewater and separatedby the crystallization device, and recover condensate water; and asludge treatment device for treating sludge, generated from thewastewater and separated by the crystallization device, into adiscardable form through centrifugation; wherein the stacked typefalling film evaporator includes: a first falling film evaporator andthe second falling film evaporator stacked in such a manner thatwastewater passes through the first falling film evaporator and thesecond falling film evaporator in order; a first pipe, connected betweena liquid storage tank of the first falling film evaporator and a vaporinlet of the second falling film evaporator, that collects vaporgenerated from the wastewater in the first falling film evaporator andsupplies the collected vapor to the second falling film evaporator; asecond pipe, connected between a liquid storage tank of the secondfalling film evaporator and a vapor inlet of the first falling filmevaporator, that collects vapor generated from the wastewater in thesecond falling film evaporator and supplies the collected vapor to thefirst falling film evaporator; and a vapor recompressor that compressesthe vapor collected in the second pipe before the vapor is supplied tothe first falling film evaporator.
 6. The zero liquid discharge systemaccording to claim 5, wherein the vapor recompressor is a thermal vaporrecompressor.
 7. The zero liquid discharge system according to claim 5,further comprising a second vapor recompressor that compresses the vaporcollected in the first pipe before the vapor is supplied to the secondfalling film evaporator.
 8. The zero liquid discharge system accordingto claim 5, further comprising a circulation pump that suppliesconcentrated wastewater exiting the second falling film evaporator to anupstream space of the first falling film evaporator.
 9. The zero liquiddischarge system according to claim 5, wherein the wastewaterpretreatment device is at least one selected from the group consistingof a caustic soda treatment tank, an alum treatment tank, a polymertreatment tank, and a settling separation tank.
 10. The zero liquiddischarge system according to claim 5, wherein the crystallizing deviceis a forced circulation evaporator.
 11. The zero liquid discharge systemaccording to claim 5, further comprising a reverse osmosis separatordisposed at an upstream side of the stacked type falling filmevaporator.
 12. The zero liquid discharge system according to claim 5,wherein the stacked type falling film evaporator is adapted to evaporatewastewater introduced through a wastewater inlet, flowing into anupstream space, and flowing in the form of a falling film along innerwalls of evaporation tubes.
 13. The stacked type falling film evaporatoraccording to claim 5, wherein each of the first and second falling filmevaporators includes: a housing having a wastewater inlet disposed at atop portion thereof and the liquid storage tank that stores concentratedwastewater disposed at a bottom portion thereof; a flow uniformitydevice that separates an upstream space from a heat exchange spacecomprising a plurality of evaporation tubes; the plurality ofevaporation tubes of a length of about 5 m to 10 m passing through theflow uniformity device such that opening portions of ends of the tubesare disposed in the upstream space; the vapor inlet that introducesvapor into the heat exchange space in order to heat the outer walls ofthe evaporation tubes; a vapor outlet that extracts vapor havingundergone heat exchange in the heat exchange space and provides theextracted vapor to the crystallization device; and a condensate waterrecovery aperture that recovers condensate water which is introducedinto the heat exchange space and concentrated through heat exchange withthe evaporation tubes, such that the wastewater introduced through thewastewater inlet flows into the upstream space and is evaporated whileflowing in the form of a falling film along inner walls of theevaporation tubes.